Generally, a power steering system provides assistant steering force to allow a driver to smoothly manipulate a steering handle, since great frictional resistance is applied to a front wheel serving as a steering wheel. The assistant steering force is determined by measuring torque applied to a torsion bar when the steering handle is steered.
In order to measure torque of the steering wheel, various torque measuring devices have been developed and used. Among them, a torque measuring device employing a magnetic field has been largely used since it has advantages in terms of a number of parts and price.
In the torque measuring device employing the magnetic field, a magnet is coupled to a steering handle that is an input shaft, one side of a stator holder fixing a stator is coupled to the input shaft through the torsion bar, and the other side of the stator holder is coupled to an output shaft of a front wheel.
Accordingly, when rotating the steering handle by applying external force to the steering handle, torque is measured by detecting a magnetic field according to a difference between a rotational angle of the magnet coupled to the input shaft and a rotational angle of the stator coupled to the stator holder to rotate together with the output shaft. The assistant steering force is determined based on the measured torque and a motor is driven according to the assistant steering force.
The currently-used torque measuring device employing a magnetic field includes a stator fixed by a second shaft connected to a steering wheel side, a magnet oppositely arranged from a stator tooth and fixed by a first shaft connected to a steering handle side, a sensing unit detecting a magnetic field generated from between the stator and the magnet, and a stator holder fixed by the stator and shafts.
The stator holder includes a stator fixture formed by a non-magnetic plastic injection molding product and a shaft fixture formed by a metal material and having a predetermined strength, where the stator fixture and the shaft fixture are mutually coupled by insert molding.
However, the stator holder has a disadvantage in that manufacturing cost is high due to the fact that the stator fixture and the shaft fixture are mutually coupled by insert molding.
Another disadvantage is that defect rates increase and plastic injection molding products are deformed by temperature when insert molding is performed because metal material and plastic material are insert-molded.
Still another disadvantage is that the metallic shaft fixture and the plastic stator fixture may be separated after the stator holder is manufactured.